1. Technical Field
The invention relates to a method and a system for surveillance and position determination of objects and/or living beings within a restricted area, such as, e.g., a room in a building. The system comprises a plurality of electronic identification chips that are attached to the objects which have to be monitored. Each chip has its own identification code (ID code) and is equipped with its own ultrasound transmitter and receiver. The system further comprises a plurality of detectors in each room for registering and interpreting the signals transmitted from the chips. The detectors are interconnected in a network and transmit the information received to one or more central units also included in the system for further processing and sorting. By means of the invention, line interference from electrical equipment, which may arise when the signals are transmitted from transmitter units to detectors, will be substantially removed, in addition to which it is possible to determine position even though the identification chips are in motion.
2. Background Information
In hospitals and other places there may be a great deal of equipment and case records which are constantly being relocated. A lot of time is wasted in finding the equipment. It is therefore expedient to have a flexible system that can determine the position of various units.
In areas where there is electronic equipment that is sensitive to electromagnetic radiation, it is inadvisable to introduce new equipment that generates such radiation, such as a transceiver based on radio waves. The measurements of the new equipment in turn will be influenced by the existing equipment.
Systems based on ultrasound will be suitable, since they will not be affected by electromagnetic radiation and will have little effect on the environment.
A weakness of known systems based on ultrasound is that the results of the measurements will be influenced by noise sources such as fluorescent tubes and computer screens. This will detract from the quality of the received signals. Another shortcoming of known systems is that they will not work when the chip transmitting the signals is in motion.
An object of the invention is to reduce the influence of noise sources to a minimum.
A second object is to perform position determination even though the chip whose position has to be determined is in motion.
There are various principles currently in use for localizing objects within a restricted area.
These include systems that employ ultrasound as signal carrier. WO-9955057, which is the applicant's own patent and of which the present invention is a further development, is an example of this. This publication describes the state of the art, and is incorporated herein in its entirety as a reference. This system, like the present invention, is also intended for surveillance and position determination of objects within a restricted area by means of chips that transmit a specific ID code in the form of ultrasound signals. The chips have continuous transmission of signals at predetermined intervals, and comprise ultrasound receivers as well as means for transmitting sound in the audible range in order to issue a warning when an attempt is made to remove a chip. The code is not transmitted after an expected period or the wrong code is transmitted. Stationary receiver units placed in each defined area are connected to a central control unit via a network and perform a two-way communication with the identification chips. In a special embodiment a specific chip can be called up from the central control unit. Calling signals are then transmitted from the stationary receiver units, and the chip with the correct ID replies. The receiver receiving the strongest signal indicates in which defined area the chip is located.
As stated above, WO-9955057 concerns a system that can localize chips to a specific room. A weakness of the system is that it cannot determine the position of chips in different parts of a room.
Other systems, including U.S. Pat. No. 6,317,386, are also known, which employ precise measurement of time delays between a transmitter and several receivers for detecting precise localization in the cm range. The drawback of these systems is that they require precise localization of all the receivers in advance, complicated signal processing and they only work when there is an unobstructed view between transmitter and receivers.
Another shortcoming of communication systems of the type based on ultrasound is that they are sensitive to line noise from, for example, computer screens, TV monitors and electronic ballast circuits in fluorescent tubes. These may transmit a constant tone between 20 and 50 kHz, and if the frequency is close enough to one of the frequencies employed in chips, it will create problems for detection or a serious degree of uncertainty in the measurements.
A problem with known solutions for detecting units that are transmitting ultrasound signals arises when they are in motion. This prevents the signals from being interpreted due to Doppler shift.
Another problem with the prior art arises when there are detectors in neighboring rooms where the doors are open. In this case several detectors can hear a signal from a chip. To obtain reliable position determination is therefore no easy matter.